Roaming occurs in a wireless network when a mobile device, such as a wireless phone, personal digital assistant (PDA), or other wireless device connects to a network other than that to which it is subscribed. Many wireless-network providers have contracted with each other to allow customers of a first network to use the services of a second network for a fee. By such agreements, the wireless-network providers pay each other for their subscribers' usage of other wireless networks.
By negotiating agreements with other wireless networks providers a single provider is able to offer continuity of service over a larger area without having to build out its own wireless network. A provider may thus, weigh the costs of paying roaming fees to other providers versus costs such as building out new wireless antennas and towers.